Adjust min serviceaccounttoken expiration to 10min

[gauravpriyolkar]

Issue #: #155

Description of changes:
The desired expiration time for serviceaccount tokens can be set via the eks.amazonaws.com/token-expiration annotation on the serviceaccount.

Currently, the 1hr minimum overrides token requests for shorter expiration: https://github.com/aws/amazon-eks-pod-identity-webhook/blob/master/pkg/validation.go#L17

This change aligns minimum serviceaccount token expiration to 10min to match the minimum permissible at apiserver: https://github.com/kubernetes/kubernetes/blob/master/pkg/apis/core/validation/validation.go#L1148

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[DanielCKennedy]

make test is failing. Can you update rawPodMinTokenExpiration.pod.yaml to change the expectedPatch on line 11 to have "expirationSeconds":600 instead of 3600

https://github.com/aws/amazon-eks-pod-identity-webhook/blob/master/pkg/handler/testdata/rawPodMinTokenExpiration.pod.yaml#L11

[tingrui-AWS-EKS-auth]

Test Case:

1. Test image building.
2. Update pod-identity-webhook manifest in the cluster with new image. Confirm pod-identity-webhook start workin without errors.
3. Follow the EKS Walkthrough in the README file, to create node role, service account. Set the token-expiration of pods from 3600 to 600 seconds.
4. Create node, and pods using service account. Get pod information, comfirm the token-expiration is 600s instead of 3600.
5. Exec into pods, test aws command like aws iam list-roles, to check the pod working normally.

[prateekgogia]

LGTM!